Degradation of passive layers of iron studied by conversion electron Moessbauer spectroscopy

Integral electron Moessbauer spectroscopy (ICEMS) and additionally some electrochemical methods were used to characterize the passivation process of iron (low carbon steel) in sulfate, sulfate + sulfite (a possible model solution of acid rain) solutions and in phosphate buffer. The phase compositions and thicknesses of the passive layers formed due to the electrochemical polarizations were analyzed in dependence on the duration of the anodic passivations and on the pH of the used electrolytes. The passive layer, as determined from the Moessbauer spectra, consists mainly of γ-FeOOH, however in sulfite containing sulfate aqueous solution at pH 3.5 Fe2C and despite ex-situ circumstances FeSO4 x H2O was detected after the shortest polarization time. The film thickness, which was found to grow nearly with polarization time in pure sulfate solution and in phosphate buffer, reached a maximum of 60-100 nm (depending on pH) in sulfate +sulfite solution after a passivation time of about 4 hours. It has been proved, that HSO3- ion, which is contained by acid rain, initiate pit formation under acid conditions and so enforces the corrosion of iron. The experimental results furthermore suggest, that not the whole oxidic layer is responsible for the passivity but only a very thin intermediate layer formed between an inner oxide layer of a cubic structure and the rhombic oxide (γ-FeOOH) cover. (author) 7 refs.; 7 figs.; 2 tabs